Scheduling by means of Time Division, TD, is an important parameter for achieving high spectral efficiency in the uplink in WCDMA systems. In TD scheduling of the so called Enhanced Uplink, EUL, only a certain amount of users at a time in a cell are allowed to transmit payload data on the E-DCH Dedicated Physical Data Channel, abbreviated as E-DPDCH, in a Transmission Time Interval, a TTI, while the other users in the cell are only allowed to transmit control data in the uplink in that particular TTI.
A user, also referred to as a Mobile Terminal, an MT, thus transmits payload data in certain TTIs and control data in other TTIs. An MT which transmits data on the E-DPDCH in a particular TTI will in this text be referred to as an active MT, and a MT which doesn't transmit data on the E-DPDCH in a TTI but only transmits control data on control channels, or which makes no transmissions at all in a TTI, will here be referred to as a passive MT. Naturally, an active MT is also arranged to transmit control data on control channels in its active state, so that an MT may transmit both payload data and control data in its active state.
Which state an MT is in is decided by the controlling node, the NodeB, of the cell that the MT is in. In addition, the output power used by an MT in its uplink transmissions is also decided by the NodeB by means of transmitting so called Transmit Power Control Commands, TPC commands, to the MTs in the cell.
Due to non-orthogonality of UL channels among different MTs, an active MT suffers less interference from transmissions from passive MTs than the interference suffered by passive MTs from the active MT. As a consequence, the interference which an MT perceives changes abruptly at transitions between the active and the passive state, with the interference typically being higher in the passive state.
Thus, as explained, a passive MT suffers more interference than an active MT, so an MT has to be controlled by the NodeB to raise its transmission power substantially when going from the active to the passive state in order to maintain a SIR close to the SIR target. When a passive MT turns to active, the SIR is well above SIR target, and the transmission power of the MT is reduced by the NodeB, since there is less interference. In this manner, the transmission power control of the MTs as controlled by the NodeB may lead to power peaks in TD scheduling scenarios. The behavior of increasing and decreasing the transmit power as controlled by the NodeB is repeated as the state of the MT changes from active to passive throughout the session.
High transmission power of passive MTs will increase the Rise over Thermal, RoT, in the cell, which becomes a drawback from a network performance perspective. Moreover, an MT is likely to transmit with too high power (exceeding SIR target) when entering an active TTI.